The sickling of erythrocytes in Sickle Cell Disease (SCD) is the result of a single amino-acid mutation, .beta.6 Glu to Val!, which leads to the polymerization of hemoglobin S (.alpha..sub.2 .beta..sup.S.sub.2) in the tense (T), deoxygenated state. Long, multi-stranded fibers form within red blood cells (RBCs) of patients with SCD. The main fibers are made of 14 twisted strands associated by pairs (the Whisher-Lowe double-strand). The .beta.6 Val mutation is required to initiate stable lateral contacts with another tetramer in the double-stranded unit. Only one .beta.6 Val mutation per .alpha..sub.2 .beta..sup.S.sub.2 tetramer is involved in inter-tetramer contact, although many other amino-acid residues participate in lateral and axial contacts within the double-strand as well as between double-strands (Bunn, H. F. and Forget, B. G. (1986) Hemoglobin: Molecular, Genetic and Clinical Aspects, (W. B. Saunders Company, Philadelphia); Bunn, H. F. (1994) in The Molecular Basis of Blood Diseases, Second Edition, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Majerus, P. W. and Varmus, H. (W. B. Saunders Company, Philadelphia), pp. 207-256; Dickerson, R. E. and Geis, I. (1983) Hemoglobin: Structure, Function, Evolution, and Pathology (The Benjamin/Cummings Publishing Company, Inc., Menlo Park, Calif.);. Schechter, A. N., Noguchi, C. T. and Rodgers, G. P. (1987) in The Molecular Basis of Blood Diseases, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Leder, P. and Majerus, P. W. (W. B. Saunders Company, Philadelphia), pp. 179-218).
Mixing .beta..sup.S with the other normal human .beta.-like globin chains (.beta..sup.X), i.e. .beta.-, .delta.- or or .gamma.-, results in an equilibrium between symmetrical (.alpha..sub.2 .beta..sup.S.sub.2 and .alpha..sub.2 .beta..sup.X.sub.2) and hybrid (.alpha..sub.2 .beta..sup.S .beta..sup.X) tetramers, because interactions at .alpha./.beta. subunit interfaces within the same hemoglobin tetramer (packing and lateral contacts) are similar among .beta.-like globin chains. Symmetrical tetramers that do not have the .beta.6 Val mutation (.alpha..sub.2 .beta..sup.X.sub.2) are incorporated into the fibers very poorly. The .alpha..sub.2 .beta..sup.S .beta. hybrid tetramers are capable of copolymerizing with hemoglobin S, because only one .beta.6 Val residue per tetramer is required for inter-tetramer contact, and the other lateral and axial contacts are formed efficiently with the trans .beta. subunit. In contrast, .alpha..sub.2 .beta..sup.S .gamma. and .alpha..sup.2 .beta..sup.S .delta. hybrid tetramers are poorly if at all incorporated, because trans .delta.- and .gamma.- globin chains are unable to form important contacts within the S fiber even when the .beta.6 Val residue of the .beta..sup.S subunit is aligned appropriately. This phenomenon is believed to explain why .gamma.- and .delta.- globins are much stronger inhibitors of sickling than .beta.-globin, in vitro. In addition, interaction of these hybrid tetramers with hemoglobin S without successful copolymerization is expected to delay the polymerization process in vivo, so that RBCs return to the lung to get reoxygenated before significant sickling has occurred (Benesch, R. E., Edalji, R., Benesch, R. and Kwong, S. (1980) Proc. Natl. Acad. Sci. USA, 77, 5130-5134; Cheetham, R. C., Huehns, E. R. and Rosemeyer, M. A. (1979) J Mol. Biol., 129, 45-61; Sunshine, H. R., Hofrichter, J. and Eaton, W. A. (1979) J. Mol. Biol., 133, 435-467; Bunn, H. F. and Forget, B. G. (1986) Hemoglobin: Molecular, Genetic and Clinical Aspects, (W. B. Saunders Company, Philadelphia); Bunn, H. F. (1994) in The Molecular Basis of Blood Diseases, Second Edition, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Majerus, P. W. and Varmus, H. (W. B. Saunders Company, Philadelphia), pp. 207-256; Dickerson, R. E. and Geis, I. (1983) Hemoglobin: Structure, Function, Evolution, and Pathology (The Benjamin/Cummings Publishing Company, Inc., Menlo Park, Calif.) Schechter, A. N., Noguchi, C. T. and Rodgers, G. P. (1987) in The Molecular Basis of Blood Diseases, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Leder, P. and Majerus, P. W. (W. B. Saunders Company, Philadelphia), pp. 179-218).
The actual inhibitory effect of .delta. chains has not yet been assessed in vivo, because the .delta.-globin gene is always expressed at very low levels in human RBCs. In contrast, there is a strong correlation between high expression levels of .gamma. chains and a lower propensity for sickling in vivo, as observed for instance in certain forms of SCD associated with hereditary persistence of fetal hemoglobin (HPFH) (Bunn, H. F. and Forget, B. G. (1986) Hemoglobin: Molecular, Genetic and Clinical Aspects, (W. B. Saunders Company, Philadelphia); Bunn, H. F. (1994) in The Molecular Basis of Blood Diseases, Second Edition, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Majerus, P. W. and Varmus, H. (W. B. Saunders Company, Philadelphia), pp. 207-256; Dickerson, R. E. and Geis, I. (1983) Hemoglobin: Structure, Function, Evolution, and Pathology (The Benjamin/Cummings Publishing Company, Inc., Menlo Park, Calif.);. Schechter, A. N., Noguchi, C. T. and Rodgers, G. P. (1987) in The Molecular Basis of Blood Diseases, eds. Stamatoyannopoulos, G., Nienhuis, A., W., Leder, P. and Majerus, P. W. (W. B. Saunders Company, Philadelphia), pp. 179-218).
In addition, drugs known to derepress partially .gamma.-globin gene expression, such as hydroxyurea and butyrate derivatives, are clearly beneficial to SCD patients. However, these approaches do not represent a definitive cure and have raised legitimate concerns regarding their potential long-term consequences, which include teratogenic and oncogenic effects for hydroxyurea and neurotoxicity and multiorgan damage for butyrate. In addition, drug induced .gamma.-globin expression might be largely restricted to F cells, so that non-F cells may still sickle in SCD (Stamatoyannopoulos, G. and Nienhuis, A., W., (1994) in The Molecular Basis of Blood Diseases, Second Edition, eds. Stamatoyannopoulos, G., Nienhuis, A. W., Majerus, P. W. and Varmus, H. (W. B. Saunders Company, Philadelphia), pp. 107-155).
New therapies for treating a subject afflicted with sickle cell disease are needed.